Grain growth 1 to 4 days

Embryo sac: events inside


Nuclear division is synchronous; the nuclei are regularly spaced around the endosperm syncytium. At a higher magnification we see that the nuclei from 'free division' have migrated to the wall of the embryo sac surrounding the large central vacuole.
Transverse section at a higher magnification showing more detail of the embryo sac wall. In the upper part of this section the individual layers of the embryo sac wall are well preserved but have delaminated due to a fixation artifact.
This median transverse section of a whole grain at 2 days after fertilization shows the antipodal cells on the ventral side of the embryo sac. The antipodal cells fuel the 'free nuclear division' stage that creates the coenocytic endosperm. They may also release enzymes which partially digest the surrounding nucellar tissue. The bulky maternal pericarp is still the largest structured tissue.
Longitudinal section of the whole grain 2 days after fertilization. The central embryo sac already contains specialist cells. The first divisions of the second polar nucleus produce two types of endosperm cell. The cellular endosperm, which has cell walls, will support the growth of the new embryo for about 14 days. The embryo is tiny compared with the other cells in the embryo sac. The coenocytic endosperm is laid down for the support of the embryo when it germinates to become a new plant. The antipodal cells are prominent at this time.
At 3 days after fertilization the antipodal cells are less prominent. The first nuclei produced in 'free division' migrate to the edges of the embryo sac. There are no cell walls associated with the coenocytic endosperm nuclei at this time
The cellular endosperm supports the growth of the embryo at 3 days after fertilization. The antipodal cells degenerate as they fuel the 'free nuclear division' stage of the coenocytic endosperm. The nuclei of the coenocytic endosperm are not surrounded by cell walls at this stage and move freely inside the embryo sac.
Transverse section at 4 days after fertilization. The 'free division' endosperm nuclei come to rest at the nucellus.
The grain grows lengthwise and the antipodal cells decline further, 4 days after fertilization. The growth of the embryo sac parallels the growth of the surrounding maternal tissues. The rapid extension of the embryo sac during this stage is achieved by cell growth (rather than cell multiplication) in the inner and outer integuments and in the nucellus.
Longitudinal section at higher magnification 2 days after fertilization.